The present invention generally relates to orthogonal frequency-division multiple access (OFDMA) systems, and more particularly, the present invention relates to methods for constructing frame structures in OFDMA systems.
Orthogonal Frequency Division Multiple Access (OFDMA) is a multiple access scheme for transmitting data in different subcarriers in a channel, wherein the data may come from different users and may be transmitted in disjoint subsets of sub-channels in a transmission bandwidth. The orthogonality property among the subcarriers may allow simultaneous transmission of data from different users without interference from one other. The multiple access scheme of the OFDMA may generally be applied in various communication systems, such as those defined in IEEE standard 802.16e (“legacy system” hereafter) and IEEE standard 802.16m (“new system” hereafter). The new system defined in the IEEE standard 802.16m may be required to provide enhanced spectrum efficiency, higher speed tolerance and full backward compatibility with the legacy system defined in the IEEE standard 802.16e.
FIG. 1 is a diagram illustrating an OFDMA frame structure under the IEEE 802.16 standard. Referring to FIG. 1, the frame structure may include a downlink sub-frame (DL sub-frame) 16 and an uplink sub-frame (UL sub-frame) 18. The UL sub-frame 18 may follow the DL sub-frame 16 in time domain with a transmit/receive transmission gap (TTG) 17 from the DL sub-frame 16. Moreover, the frame structure may be separated from the next frame structure, led by a preamble 10-2, by a receive/transmit transmission gap (RTG) 19.
The DL sub-frame 16 may include a preamble 10-1, a frame control header (FCH) 11, a downlink map (DL-MAP) 12, a downlink burst (DL burst#1) 13 and a data region (DATA) 14-1. The UL sub-frame 18 may include a ranging sub-channel 15 and a data region (DATA) 14-2. Since the DL-MAP 12 may be used to identify the division or structure of the DATA 14-1 in the DL sub-frame 16, it may be desirable to integrate the OFDMA frame structure of an old OFDMA system with that of a new OFDMA system by using the DL-MAP 12 to define different zones in the DATA 14-1 and DATA 14-2 of the frame structure for data of the old OFDMA system and data of the new OFDMA system.
FIG. 2 is a diagram illustrating a placement of guiding signals (or pilot symbols) 24-1 for time-domain and frequency-domain OFDMA signals under the IEEE 802.16 standard. Referring to FIG. 2, upper and lower frequency bands may serve as guard bands 22-1 and 22-2, respectively, which may not be used to carry information. The placement of information may include a first part and a second part. For example, the first part of the placement includes a preamble 10′-1 having a fixed length, and the second part of the placement includes data and guard intervals between an upper row and a lower row of the data interlaced with the pilot symbols 24-1, represented by blocks marked with “X”. In some applications such placement of information may be inflexible to bandwidth scaling due to the fixed-length preambles 10′-1 and/or 10′-2 and the often unusable guard bands 22-1 and 22-2. Moreover, the placement may be susceptible to a Doppler effect in a high mobility scenario because the placement may be usually designed with a relatively large symbol period, which in turn may induce relatively short carrier spacing and less dense pilot symbol placement. Moreover, the limitation on pilot symbol placement may cause channel estimation error at a receiving end because of insufficient information provided for channel estimation.